The long-term goal of this project is to understand the roles of vestibular otolith and canal function in the control of human posture and balance. A biomechanical model of postural motor coordination will be expanded to include vestibular system dynamics. This model will be used to generate specific predictions concerning the performance of humans responding to support surface displacements, which will be tested experimentally in healthy humans and patients with vestibular disorders. Experimental results will be compared to computer simulations to refine and extend the model. The specific aims of the proposed work are: I. To develop a computational model of the sensory to motor transformation relating vestibular information and postural control. The frequency response characteristics of human balance control and vestibular sensory dynamics will be incorporated into a recently developed biomechanical model of human postural control. II. To quantify the frequency response characteristics of human postural control. Leg, trunk, and head motions, joint torques, surface reactive forces, and muscle activation patterns will be measured in healthy humans responding to predictable and unpredictable sinusoidal support surface displacements. These experimental results will provide the frequency response characteristics of the human postural control system, including strategy selection, which will be incorporated into the model of human postural control. III. To determine the extent to which postural motor coordination and hence, postural objectives, can be altered by volitional intent and by sensory conditions. Using optimization techniques, the model described in Specific Aim I will be used to make predictions about changes in the frequency response characteristics of healthy humans in response to changes in the postural control objectives (i.e., control of upright trunk vs. limitation of center of mass movement) or changes in sensory information (i.e., alterations in visual, somatosensory, and/or otolith information about body sway). These predictions will be tested experimentally, and the results will be used to refine the model. IV. To examine how postural movements are altered by vertical semicircular canal and/or otolith dysfunction. Using optimization techniques, the model will be used to make specific predictions about the postural motor coordination of patients with vestibular canal and/or otolith dysfunction. These predictions will be tested experimentally by studying the postural coordination of patients with canal and/or otolith dysfunction identified in Dr. Peterka's project. Orientation of static posture to altered gravito-inertial forces in a slow rotating room will also be tested in normals and vestibular loss subjects. The studies proposed here will significantly expand our understanding of the role of vestibular information in postural control in health, disease, and adaptation to microgravity. This understanding will lead directly to new amelioration strategies for the posture and balance problems of both vestibular patients and astronauts.